A transcritical process traditionally used in automobile air conditioning applications (solid lines, process B) is given in a pressure/enthalpy diagram shown in FIG. 2, whereby the process is described by a compression (state 1″-state 2″), a heat output to an external heat ex-changer (state 2″-state 3″), the high-pressure side of an internal heat exchanger (state 3″-state 4″), an irreversible isenthalpic expansion (state 4″-state 5″), an internal heat exchanger (state 5″-state 6″), and the low-pressure side of the internal heat exchanger (state 6″-state 1″).
A problem is that at high ambient temperatures the coefficient of performance of the process decreases relatively distinctly compared with current refrigerants, or the alternatives thereof.
The thermodynamic analysis of the process indicates that a work-doing expansion of the carbon dioxide can, depending on the design, at least compensate for the disadvantage. To achieve this, the irreversible expansion 4″-5″ is replaced with a reversible expansion doing work (schematically, from 5′-6′).
Such an air conditioning unit 1, operatable with carbon dioxide, for mobile applications is described in DE 19841686 C2, DE 10013191 C1, and DE 10220391 A1, whereby refrigeration/air conditioning units are given having an expansion machine in form of a toothed gear machine. The embodiments are one-stage refrigerant processes, whereby the air conditioning unit includes a carbon dioxide based refrigerant circuit which includes, in direction of flow, a compressor, a gas cooler or liquefier, an expansion machine in form of a toothed gear machine (externally toothed, helically toothed), and an evaporator, the components connected to each other through lines. In the refrigerant circuit, the carbon dioxide is expanded doing work into the two-phase region, whereby the starting point of the expansion may be supercritical or subcritical. Depending on the ambient temperature, the unit can be operated in the air conditioning mode or heat pump mode. The energy obtained during the expansion process of the toothed gear machine by the work-doing expansion is used as, for example, mechanical or electrical power to drive the compressor. The mechanical integration into the main compressor is relatively expensive and the efficiency chain is critical for the conversion into electrical power.
Another transcritical process traditionally used in stationary refrigeration/air conditioning applications, according to the state-of-the-art, is given in a pressure/enthalpy diagram, FIG. 2, (dashed lines, process A) whereby the process is described by a compression (state 1′-state 2′), a heat output to a first external heat exchanger (state 2′-state 3′), a subsidiary compressor stage (state 3′ to 4′), a second external heat exchanger (state 4′ to 5′), an expansion stage 5′ to 6′, directly driving the subsidiary compressor stage, and an internal heat exchanger (state 5′-state 6′).
Such an air conditioning unit 1, operatable with carbon dioxide, for stationary applications, according to FIG. 1, is described in DE 19813220 C2 but is different in that an expansion-compression machine is provided established as a free piston machine. The expansion-compression machine given therein is, as to DE 10242271 B3, further developed in that the expansion is configured multi-stage with the configuration of the free piston machine maintained. The air conditioning unit 1 includes a main compressor 2, a first external heat exchanger 3, a subsidiary compressor 4, a second external heat exchanger 5 and an expansion machine 6, and an internal heat exchanger 7. They are connected to each other, in this order, to build a circuit 8 for the carbon dioxide. The expansion ma-chine 6 and the subsidiary compressor 4 are configured as an expansion-compression machine 9. The expansion-compression machine 9 is a piston-type expansion machine configured to the free piston design, with work-doing expansion of the carbon dioxide, the energy of which is used to compress the carbon dioxide in the high-pressure stage of the expansion-compression machine 9. A multi-stage expansion is provided, whereby the expansion can be managed to occur using several expansion machines with double-acting cylinders, whereby the pistons of the expansion machines are mechanically linked through a piston rod to the piston of the subsidiary compressor 4. The work-doing expansion occurs in the expansion machine 6, the compression in the high-pressure stage of the subsidiary compressor 4 provided. The working principle of the refrigeration unit 1 is that the carbon dioxide is simultaneously subjected to a work-doing expansion in the expansion machines 6 and a compression in the subsidiary compressor 4.
A problem is that the use of the described expansion-compression machine in mobile applications such as automobiles implies assembly and operation complications due to the design and the dimensions resulting therefrom, as the automobile industry aims at the miniaturization of the automobile components.
A refrigeration machine with a second compressor stage is described in Lorentzen, G.: The use of natural refrigerants—a complete solution to the CFC/HCFC predament, Proceedings of conference New Application of Natural Working Fluids in Refrigeration and Air Conditioning, Hannover, 10-13.5.94, 23/36, whereby the air conditioning unit contains a first compressor, a first gas cooler, a second compressor, a second gas cooler, an expansion machine and an evaporator, which are connected in series to each other forming a closed refrigeration circuit. The compressor and expansion machine, both configured as piston machines, are connected to a drive motor and are driven by said motor.
In DE 102005017623 A1, only the increase of the specific refrigeration output is used with the expansion doing work. The mechanical expansion work available is transformed into friction heat which heats the refrigerant circuit. Using this method improves the coefficient of performance in the air conditioning mode, whereas the high potential of work-doing expansion is not utilized.